|
Holostei are bony fish that retain basal characteristics inherited from an early time in their evolution. There are eight species divided among two orders, the Amiiformes represented by a single living species, the bowfin (''Amia calva''), and the Lepisosteiformes, the gars. There are more species to be found in the fossil record. Holostei share with other non-teleost ray-finned fish a mixture of characteristics of teleosts and sharks. In comparison with the other group of non-teleost ray-finned fish, the chondrosteans, the Holostei are closer to the teleosts and further from sharks: the pair of spiracles found in sharks and chondrosteans is reduced in holosteans to a remnant structure: in gars, the spiracles do not even open to the outside;〔(Ontario. Game and fish commission )〕 the skeleton is lightly ossified: a thin layer of bone covers a mostly cartilaginous skeleton in the bowfins. In gars, the tail is still heterocercal but less so than in the chondrosteans. Bowfins have many-rayed dorsal fins and can breathe air like the bichirs. The gars have thick ganoid scales typical of sturgeons whereas the bowfin has thin bony scales like the teleosts. The gars are therefore in this regard considered more primitive than the bowfin.〔(【引用サイトリンク】 title=Holostei )〕 The Holostei is often regarded as paraphyletic; as a result this infraclass is often not used, the two orders contained in it being treated instead simply as members of the sub-class Neopterygii without any further taxonomic ranking. However, in recent work mitochondrial analysis of DNA seems to support the recognition of Holostei as a clade, so the question is not yet resolved. Nearly all living bony fishes are teleosts. The name ''Holostei'' derives from the Greek, ''holos'' meaning whole and ''osso'' (osteo) meaning bone: a reference to their bony skeletons. ==References== 〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Holostei」の詳細全文を読む スポンサード リンク
|